1. Field of the Invention
The present invention relates to an image forming apparatus using an electrophotographic process such as, for example, a copying machine, a printer, a facsimile or other image forming apparatus.
2. Related Background Art
Conventionally there are various color image forming apparatuses having a plurality of image forming sections, each of which forms a toner image having a color different from colors of other sections and transferring the toner images on an identical recording material with being sequentially superimposed to obtain a color image. Among these image forming apparatuses, a color copying machine with a polychromatic electrophotographic process is frequently used.
An example of this type of color electrophotographic copying machine is briefly described below by referring to FIG. 14. As shown in FIG. 14, the color electrophotographic copying machine includes a conveying belt 108 suspended between a pair of rollers 111 and caused to travel in a direction indicated by an arrow c by a driving source which is not shown and is provided with four image forming sections Pa, Pb, Pc, and Pd arranged above the conveying belt 108. The image forming sections have the same configuration and therefore the configuration will be outlined below by giving an example of the image forming section Pa of a first color.
The image forming section Pa has a drum-shaped image bearing member, that is, a photosensitive drum 101a rotating in a direction indicated by an arrow A and arranged in the vicinity of the conveying belt 108. After a photoconductive layer of a surface of the photosensitive drum 101a is uniformly charged by a primary charge 115a, a light image 116a of an yellow component of an original image is exposed to form an electrostatic latent image on the photosensitive drum 101a. The latent image shifts to a position of a developing device 103a by a rotation of the photosensitive drum 101a and developed by yellow toner supplied by the developing device 103a at the position, so that the latent image is visualized as a yellow toner image.
With a rotation of the photosensitive drum 101a, the yellow toner image reaches a transfer position where a blade-shaped transfer charger 104a having a conductive blade is disposed. At the same timing, a recording material, which is not shown, is supplied from the conveying path 112 onto the conveying belt 108 and then conveyed to a transfer position by the conveying belt 108. In addition a transfer bias is applied to the transfer charger 104a, by which the yellow toner image on the photosensitive drum 110a is transferred to the recording material.
Subsequently, a cleaning blade 130 of a cleaning unit cleans residual toner on the photosensitive drum 101a, by which the photosensitive drum 101a is prepared to enter the next image forming process. On the other hand, the recording material to which the yellow toner image is transferred advances to the next second-color image forming section Pb by conveyance with the conveying belt 108.
The second color image forming section Pb has the same configuration as the first color image forming section Pa, and therefore in the same manner as the above a latent image is formed on a photosensitive drum 101b, the latent image is developed by magenta toner, the obtained magenta toner image is transferred and superimposed on the yellow toner image on the recording material in its transfer portion. In the same manner, a cyan toner image and a black toner image are formed on the photosensitive drums 101c and 101d in the image forming sections Pc and Pd, respectively, and they are sequentially transferred and superimposed on the recording material by transfer chargers 104c and 104d, thereby achieving a color image having 4-color toner images superimposed on the recording material.
The recording material to which the 4-color toner images have been transferred is submitted to charge elimination and then separated from the conveying belt 8 by a separation charge-eliminator 161, conveyed to a fixing device 107 having a pair of a fixing roller 107a and a pressure roller 107b, and pressed and heated in a nip portion of the rollers 107a and 107b normally heated at a predetermined temperature for fixing. This mixes the colors of the toner images and fixes them to the recording material, by which a full-color permanent image is obtained and then the recording material is ejected to the outside of the copying machine.
After the recording material is separated from the conveying belt 106, an inside charge-eliminator 113 and an outside charge-eliminator 114 eliminate charges on the conveying belt received at the transfer and further a cleaning blade 120 and a backup roller 121 arranged in the downstream in a traveling direction remove rubbish or the like such as fog toner, scattered toner, or paper dust adhered to the surface of the conveying belt so as to clean the surface in preparation for the next image formation.
Additionally, in the image forming apparatus for forming a color image in the multi-transfer process, there has been used a single-layer resin belt made of polyethylene terephthalate resin, polycarbonate resin, or the like having a high resistance, specifically a volume resistivity of 1015 xcexa9xc2x7cm or greater for a purpose of attracting the recording material tightly for the transfer belt. For a transfer charging member, a transfer blade is recently used which enables a transfer electric field to be narrow in the transfer area in order to reduce a poor image such as a scatter at the transfer.
The transfer belt 108 used here is required to have various performances in order to convey the recording material stably and to achieve the multi-transfer of the 4-color toner image without any poor image.
It is a first reason for wide use of the transfer belt in the polychromatic image forming apparatus as described in this embodiment that the recording material is stably conveyed. In other words, in an image forming apparatus in which a recording material passes a plurality of image forming sections during the multi-transfer of respective colors, the image forming sections form images of their own colors in accordance with a timing when the recording material is conveyed and the images are sequentially transferred to the conveyed recording material. Unless the recording material is electrostatically attracted to the transfer belt nor fixed by any means during conveyance, the recording material is not stably conveyed between the plurality of image forming sections, leading to misregistered images of the respective colors at the transfer or to any recording material jammed in the worst case. In this embodiment, the recording material supplied from the registration roller is integrated with the transfer belt to pass the transfer nip in the first image forming section, by which the toner image in the first image forming section is transferred, and electric charges are supplied to the both sides of the recording material and those of the transfer belt, by which the recording material and the transfer belt are electrostatically attracted to each other. At this point, electric resistance conditions of the transfer belt relate to the electrostatic attraction. In general, the electrostatic attracting force is generated by an electric field given to substances having different permittivities. If a transfer belt having a low electric resistance is used, however, electric charges given to the transfer belt surface are easily eliminated, by which the attracting force may be reduced.
Therefore, to obtain a conveyance effect of a stable electrostatic attracting force, it is preferable to use a transfer belt having a volume resistivity of about 1010 xcexa9xc2x7cm or higher and there is an example of a use of an insulating substance. The use of the insulating substance or a material having a high resistivity for the transfer belt in this manner, however, easily causes a separation electric-discharge on separating the recording material P which has completed to be processed with the 4-color multi-transfer from the transfer belt, and therefore the separation charge-eliminating function need be actively used in the separation section.
Next, the resistance conditions of the transfer belt significantly relate to a transfer effect. If the resistance of the transfer belt is low, there may be problems such as electric interference, a small-sized recording material, or a scatter. If high-resistant material such as an insulating substrate is used, a high voltage is applied, by which abnormal discharge easily occurs in various places, leading to an increased possibility of image degradation.
The electric interference is additionally described by using FIG. 14, for example. In FIG. 14, toner images on the respective photosensitive members formed by the four image forming sections are sequentially transferred by the respective transfer chargers in this configuration. If a resistance of the transfer belt is low, an electric field applied by the transfer charger corresponding to the first image forming section leaks to the second image forming section, and further to respective rollers around which the transfer belt is stretched or a driving roller, which obstructs the electric field contributing to transferring the toner image in the first image forming section. The electric interference means this phenomenon. The phenomenon may occur when using a material such as paper that has been left under a high-humidity environment.
Next, the small-sized recording material problem occurs particularly when using a sheet of the recording meterial shorter in the widthwise direction than a sheet of the maximum size on which the image forming apparatus can form an image. It will be described below by using FIG. 15.
Referring to FIG. 15, there is shown a diagram of the transfer portion of the image forming apparatus shown in FIG. 14, viewed from the traveling direction of the recording material, with a recording material having a shorter width (about xc2xd) than image forming effective lengths of the photosensitive drum, the transfer belt, and the transfer charger existing in the center portion in the widthwise direction. If the recording material having the shorter width than the effective charging length of the transfer charger enters the transfer portion in this manner, the electric load (electric resistance) against the transfer charging bias varies by an amount of the recording material between an area where the recording material exists in the transfer portion and an area where the recording material does not exist in the transfer portion, and charging ability (an applied current amount) also varies. Estimating the variation, a resistance R1 in the nip portion where the recording material exists and a resistance R2 in the nip portion where the recording material does not exists are obtained as follows:
R1=(Rd+Pb+Rc+Rp)/dx=(R+Rp)/dx
R2=(Rd+Rb+Rc)/{d(Lxe2x88x92x)}=R/{d(Lxe2x88x92x)}R=Rd+Rb+Rc
where Rd, Rp, Rb, and Rc are the resistivity per unit area for the photosensitive drum, the recording material, the transfer belt, and the transfer charger (xcexa9xc2x7cm2), respectively, d is a nip width (cm), L is an effective charging length (cm) of the transfer charger, x is a recording material width (cm), and V (V) is a given voltage applied to the transfer charger.
Assuming that V (v) is a charger voltage, current il flowing in the recording material section is obtained as follows:
il=V/R1=Vdx/(R+Rp)xe2x80x83xe2x80x83(1)
On the other hand, a relation between a synthetic resistance RO and a total current I(A) pan be expressed as follows:
V=ROI
RO=R1R2/(R1+R2)=(R+Rp)/d/Lx {1+Rp/R(1xe2x88x92x/L)}xe2x88x921
Substituting these for the equation (1),
il=xI/Lx {1+Rp/R(1xe2x88x92x/L}xe2x88x921xe2x80x83xe2x80x83(2)
xe2x80x83O less than x less than L
Therefore, the current In per unit length In the widthwise direction applied to the recording material is obtained by:
In(x)=I/Lx{1+Rp/R(1xe2x88x92x/L)}xe2x88x921xe2x80x83xe2x80x83(3)
O less than x less than L
The equation (3) is a monotone increasing function on x. Therefore, when using a recording material having a shorter width than the effective charging length L of the transfer charger, the effectively applied current value is decreased. Additionally it is apparent from this function that, if Rp/R is relatively great, in other words, if the recording material is narrow In its width and thick with the high resistivity in the thickness direction, this decrease is remarkable. In this manner, if the current applied to the recording material is decreased, a toner image may be poorly transferred without achieving a complete transfer to the recording material depending on the degree of the decrease. There is almost no problem of this phenomenon if the resistance except the paper resistance Rp, i.e., R (=Rd+Rb+Rc) is large enough relative to Rp. Note that, however, the transfer belt resistivity Rb need be sufficiently high as well as the photosensitive drum resistivity Rd and the charger resistivity Rc.
Furthermore, scattering is a phenomenon that toner on a transfer portion or a transferred toner image after transfer processing is scattered over a white area. The toner image after the transfer is electrostatically retained on the recording material by electric charges given by the transfer charger on the back side of the transfer belt via the recording material and the transfer belt. If the transfer belt resistivity is low, however, its retaining force is reduced to be unstable.
Also at the transfer, if a transfer belt having a low resistance is used, a transfer electric field becomes wide and therefore a contribution of the transfer electric field occurs in the upstream of the transfer nip, thereby the transfer electric field is applied before the toner image enters the transfer nip. If the transfer electric field is applied before the recording material is brought into contact with the toner image in this manner, the toner image travels in the air, which may result in scattering of the toner image.
On the other hand, if the transfer belt resistivity is high, a high voltage is applied to the transfer charger itself, which may easily cause an abnormal discharge. In particular, the abnormal electric discharge occurred in the vicinity of the transfer nip may affect the transfer of the toner image to cause a poor image.
It is an object of the present invention to provide an image forming apparatus that keeps an ability of conveying a recording material to prevent poor images from occurring.
Other objects besides the above shall be apparent to those skilled in the art from the detailed description below.